1. Field of the Invention
The invention relates to methods for mounting thermoplastic substrates to organic, dielectric films and the resulting optical filters. More particularly, it relates to methodologies for fusing, through a variety of industrial processes, thermoplastic substrates to dielectric films without delamination while improving optical transmission at the film""s refractive boundary.
2. Description of the Prior Art
Dielectric stacks have previously been constructed by sputtering layers of inorganic (metal) oxides onto polymeric substrates. The layers alternate between high and low indices of refraction, whereby incoming light encounters a partial mirror at each new layer. By creating so-called quarter wave stacks, certain wavelengths can be destructively interfered via Bragg diffraction. These constructs possess several advantages as follows: they are intimately, optically coupled to the substrate thereby providing high transmissions; they are custom engineered for the shape and curvature of each particular part. However, numerous disadvantages exist with these constructs as follows: the labor, materials and equipment costs for manufacturing are prohibitive; the mechanical coupling of metal oxide layers onto polymeric substrates is subject to delamination from flexural stress; the differing rates of thermal expansion between the metal oxides and the polymeric substrate can also cause delamination.
Recently, dielectric stacks have been manufactured from alternating layers of polymers having high and low indices of refraction, creating organic, optical, dielectric films. Several examples of these films are described in U.S. Pat. No. 5,882,774, entitled Optical Film, the contents of which are incorporated herein by reference thereto. The different polymers have similar mechanical properties and melting temperature profiles allowing hundreds of layers to be stacked and stretched to control the thickness and optically tune the film. While this allows the film to be economically manufactured, a problem exists in optically coupling the film to other components, for example, an optical thermoplastic substrate. Because the delicate film is highly susceptible to delamination, melting, burning or having its desirable transmission properties altered, previous attempts to mount the film have been limited to adhesion via optical fluids or optical adhesives. While these methods are adequate, they severely reduce the film""s high transmission characteristics, by providing refractive boundaries from film-to-adhesive and from adhesive-to-substrate.
Accordingly, it would be desirable to mount the film onto a substrate with structural and mechanical integrity while maintaining, or enhancing, the film""s transmission characteristics.
It is therefore an object of the invention to provide a variety of methods for manufacturing optical filters that incorporate organic, dielectric films.
It is a further object of the invention to provide industrial processes for fusing thermoplastic substrates to dielectric films without delamination while improving optical transmission at the film""s refractive boundary.
It is yet another object of the present invention to describe criteria for selecting chemical and material properties of thermoplastics which can be effectively utilized in the film fusing methods.
It is another object of the present invention to describe criteria for thermally fusing the film to a thermoplastic substrate based on the chosen manufacturing method.
It is a further object of the present invention to describe the optical properties of the thermally fused region.
These and other related objects are achieved according to the invention by a method for optically coupling a thermoplastic material to an outer surface layer of an organic, dielectric, optical film. A dielectric film is selected that includes (i) repeating optical layers of at least two polymers having different refractive indexes from each other, (ii) an exterior film surface, (iii) a refractive boundary along the exterior film surface, and (iv) a delamination threshold based on total thermal energy delivered to the film. Next, a thermoplastic material which is miscible with the exterior film surface is selected. The thermoplastic material is fused to the refractive boundary with thermal energy below the delamination threshold to form a polydisperse region having a higher optical transmission than the refractive boundary. The molecular weight of the thermoplastic is selected so that the melting temperature range of the thermoplastic overlaps the melting temperature range of the exterior film surface.
Fusing may be accomplished by film insert molding, where the thermoplastic is simultaneously molded into a substrate and fused to the film""s refractive boundary. The molten resin is held above the resin""s glass transition temperature in the barrel and experiences a temperature drop as it enters the cavity to below the film""s thermal delamination threshold.
Fusing may also be accomplished via laser welding where a radiation absorbing material is placed near one of the refractive boundaries and irradiating the absorbing material through the thermoplastic or the film.
Fusing may further be accomplished by extruding the thermoplastic into a substrate and bringing the film into contact with the substrate as the substrate""s temperature drops below the thermal delamination threshold.
The optical filters manufactured according to the methods of the invention, may have incorporated therein, organic absorber dyes, such as a UV absorbing dye, a visible light absorbing dye, a cosmetic dye, a laser absorbing dye, a near infrared absorbing dye, and infrared absorbing dye and combinations thereof. Add-on filters in the form of hardcoat layers, anti-reflection layers, holograms, metal dielectric stacks and combinations of these may be combined with the thermoplastic-film construct.